This study involves the genetical, cytological and molecular characterization of the while locus of Drosophila melanogaster. Comparisons of the normal gene structure with those of mutants that perturb the regulation of the locus are being made in an effort to understand the nature of the mutational changes and how they effect the function of the locus. The mutations currently under investigation result from the insertion/deletion of transposable elements near the 5' end of the gene. Some of the mutants exhibit a mosaic of normal and mutant patches in the eye that reflect a clonal expression of the white gene while others are mosaic and show a nonclonal, nonautonomous expression. The two types of mutants are related to each other by descent and the objective is to determine the nature of the mutational changes and try to relate them to the way that regulation of the locus is modified.